re: DIY Plenum and Ductwork Replacement

Laying the schedule out in my head: I'd get all the duct as reasonably close to ran as possible (basically ran next to the old duct) and I'd prefab the plenum in the attic next to where it's going to rest. Then once I have everything in place, the next available decent weather weekend, I'll pull all the old stuff, install plenum, connect ducts to defusers and new plenum. No dampers, all straight duct runs with one split. AC cools the house well enough but I know I'm losing a lot of efficiency with leaks. I plan on matching diameters to current ductwork.

Regarding flex vs hard duct, my house is a mishmash of both with original being hard, then two additions being flex. I would like to go all flex with the new stuff. Is there any downside?